A lead frame, together with a semiconductor chip, is an important element constituting a semiconductor package, and functions as both a lead for linking the inside to the outside of the semiconductor package and a support for supporting the semiconductor chip.
FIG. 1 shows the structure of a conventional lead frame.
With reference to FIG. 1, the conventional lead frame comprises a chip pad 2 for mounting a chip as a memory device thereon to maintain the chip in a static state, inner 1 and outer leads 6 for linking the chip to an external circuit, bus bars 4 formed in regions between the chip pad 2 and the inner leads 1 to ground the chip and electrically stabilize the chip, and a lead lock 5 in the form of a composite film formed of an adhesive to fix the inner leads 1.
Since the conventional lead frame must secure the regions for the bus bars 4 to ground the semiconductor chip, there is a limitation in reducing the size of the lead frame. In addition, another problem of the conventional lead frame is that the elements, such as the inner leads 1, are deformed during down setting resulting from the regions for the bus bars 4. These problems will be explained in further detail below. The chip pad 2 is formed in a region lower than regions for the inner leads 1 in order to maintain a constant height during wire bonding of a semiconductor chip mounted on the chip pad 2 to the inner leads 1. At this time, to ensure a difference in height between the chip pad 2 and the bus bars 4 of the lead frame, the region for the chip pad 2 is formed in such a manner that it has two or more steps. As a result, a difference in height between the inner leads 1 and the semiconductor chip is non-uniform, causing frequent deformation of the inner leads 1 during wire bonding between the inner leads 1 and electrodes of the semiconductor chip due to the height difference.
On the other hand, a high-temperature atmosphere is required for easy adhesion of the lead frame to the semiconductor chip. At this time, the lead lock adhered to the top of the lead frame has a coefficient of thermal expansion different from that of the lead frame. Accordingly, when the lead lock, i.e. a composite film, is adhered to the top of the lead frame to fix the lead frame and is then recovered to its original state, the dimension of the lead frame becomes inexact due to the composite film. That is to say, since the lead frame generally has a higher coefficient of thermal expansion than the composite film constituting the lead lock, it is expanded more at a high temperature. Thereafter, the composite film and an adhesive are adhered to the top of the lead frame. After completion of the processing, when the composite film is recovered to its original state at room temperature, distortion occurs between the composite film and the lead frame because the thermal shrinkage of the lead frame is different from that of the composite film. As a result of the distortion, separation between the lead frame and the chip takes place or separation between lead frames varies, thus resulting in formation of defects in the semiconductor chip.
U.S. Pat. No. 5,545,850 discloses an improvement in the adhesion of a lead lock to a molding resin wherein the lead lock includes a metallic layer. The metallic layer is used only to improve the adhesion to the molding resin, thus reducing the risk of delamination at the interface between the lead lock and the molding resin and preventing the occurrence of distortion between the lead lock and a lead frame during wire bonding. However, the function of the metal layer as a ground, power or bus cannot be expected.